1. Field
The disclosure relates generally to differential amplifiers and, more specifically, to body biasing circuitry for differential amplifiers.
2. Description of the Related Art
As the demand for bandwidth and gain requirements in serial link receivers increases, complex problems continue to rise to the forefront. For example, input signals of varying ranges often lead to non-linear operation of differential amplifier stages. These non-linear responses are caused by limited common mode range of the differential amplifier. Design engineers continually seek to solve problems relating to common mode range of their devices.
In a differential amplifier, the input common mode range refers to the range of differential input signals over which a differential amplifier maintains a linear response, including differential gain. In its simplest form, a differential amplifier has a pair of differential input transistors that receive a differential signal. Differential input signals have a common mode voltage that is the average of the differential voltage input signal received by the pair of transistors. Certain applications require a high common mode range. As common mode requirements go to extreme highs and lows, as compared to the power supply of the amplifier, biasing problems associated with the differential input transistors and current sources of the amplifier arise. These biasing problems lead to non-linear responses and inaccurate differential gain outputs of the differential amplifier.
To solve the problem of lack of input common mode range, stages have often been added to amplifiers to shift the common mode range. One example of an added stage is an active level shifter. However, adding additional circuitry causes processing speed to decrease and the size of the devices to increase. Processing speed decreasing with device size increasing creates an even greater problem for applications having increased bandwidth and gain requirements. Further, level shifting does not extend common mode range. Level shifting only shifts a default common mode range to a desired level. Ultimately, design engineers have generally had to live with the common mode range present in the devices.
Therefore, it would be advantageous to provide transistor biasing circuitry that can extend the input common mode range.